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Two Types of Brain Problems Are Found to Cause Dyslexia

Dyslexia appears to be caused by two distinct types of brain problems, a new study has found.

The researchers, from Yale, used scanning devices to examine the brains of 43 young adults with known reading disabilities while they performed reading tasks. Another group of 27 good readers were also studied.

All the subjects had been tracked for reading ability since elementary school.

One group appeared to have what the researchers called a ''predominantly genetic type'' of dyslexia.

These students had gaps in the neural circuitry that the normal readers used for the basic processing of sound and language, but had learned to enlist other parts of the brain to compensate for the difficulty. They still read slowly but can comprehend what they read.

The second group had what the researchers called a ''more environmentally influenced'' type of dyslexia. Their brains' system for processing sound and language was intact, but they seemed to rely more on memory than on the linguistic centers of the brain for understanding what they were reading. These students had remained persistently poor readers, scoring poorly on speed as well as comprehension.

The two groups of poor readers were from similar socioeconomic backgrounds and had comparable reading skills when they began school, according to the study, which was published this month in the journal Biological Psychiatry.

But there were two differences: the students who compensated for their problems tended to have higher overall levels of learning abilities, and the students whose problems persisted were twice as likely to attend what the researchers called disadvantaged schools.

The study's lead author, Dr. Sally E. Shaywitz, said the discovery that the neural systems for reading are intact in the students with the most serious reading problems came as a surprise. It also implies that their problems are more correctable than may have been thought, she said.

''The persistently poor readers have a rudimentary system in place, but it's not connected well,'' Dr. Shaywitz said. ''They weren't able to develop and connect it right because they haven't had that early stimulation.''

A large body of research has shown that intensive tutoring can correct this kind of reading problem, especially if begun while the children's brains are still developing.

''If you can provide these children early on with effective reading instruction, these children can really learn to read,'' Dr. Shaywitz said.

For the study, participants lay in a brain scanner known as a functional M.R.I. device, and peered through a periscope at monosyllabic words flashed on a computer screen. As they read, pictures were taken of what parts of their brains were doing the work, and how they were working together.

Good readers used three areas in the left side of the brain, to decode letters into sounds, fit them together to make words and process them fluently.

The readers who had compensated but still read slowly did not use the same brain regions for those tasks. Instead, they created an alternate neural pathway, reading mostly with regions on the right side of the brain -- areas not as well suited for reading, the researchers said.

Paradoxically, the poorest readers in the study used the same parts of the left side of the brain that the normal readers did to begin the reading process. But instead of connecting that work to other language centers, they then activated a portion of the front right side of the brain that is used primarily for memory to help them along.

That overreliance on memory could help explain the persistence of these poor readers' problems.

''Once the brain makes the connections it needs for certain tasks, it tends to stick with them,'' said Dr. Gordon Sherman, the executive director of the Newgrange School and Educational Outreach Center in Princeton, N.J., an expert on dyslexia who had no connection to the study. ''But those connections aren't necessarily the best ones.''

Rote-based learning of words can get a student to a certain point, but ''then it fails quite miserably; there's too much to memorize,'' Dr. Sherman said.

J. Thomas Viall, executive director of the International Dyslexia Association, said that the study's findings underscored the need for intensive educational intervention, but that more work was needed before its findings of subgroups could be translated into practical applications.

''Dyslexia is a disorder whose treatment is education,'' Mr. Viall said. He called the notion of using functional M.R.I. data to identify types of dyslexia ''exciting'' but added, ''We don't have F.M.R.I.'s in grocery stores like blood pressure machines.''

Dr. Shaywitz acknowledges that giving such tests to every child is impractical, but says researchers will now begin to use the brain connectivity studies to develop other kinds of diagnostic tests.

''It is possible from all that we've learned about the science of reading to identify all children who are at risk for experiencing reading difficulty,'' she said.

The next step, she added, will be to design early strategies that are tailored to each child's particular type of disability.